1. Field of the Invention
The present invention relates to a process for brominating a side chain of m-phenoxytoluene by using a polyhalogenated ethane as a brominating agent in the presence of a catalyst of amorphous carbon.
2. Description of the Prior Art
m-Phenoxybenzyl bromide and m-phenoxybenzylidene dibromide obtained by brominating a side chain of m-phenoxytoluene can be easily derived into alcohol components of synthetic pyrethroid insecticides which are recently found such as m-phenoxybenzyl alcohol and m-phenoxybenzaldehyde by an esterification followed by a hydrolysis or only hydrolysis or the Sommelet reaction of the product. Therefore, the products are quite useful as intermediates for synthetic pyrethroid insecticides.
It has been reported that a bromination of phenyl nucleus is resulted together with a bromination of the side chain of m-phenoxytoluene in the process for brominating the side chain of m-phenoxytoluene by using bromine as the brominating agent.
The production of phenyl nucleus-brominated compounds causes a reduction of a yield and also causes an industrial trouble in view of difficulty of the separation of the by-products from the side chain brominated compound as the object product.
It has been reported that the bromination of the side chain of m-phenoxytoluene with bromine at 220.degree. C. or higher in the presence of phosphorus halide as described in U.S. Pat. No. 4,014,940 and in the radiation of ultraviolet rays at 180.degree. C. or higher as described in U.S. Pat. No. 4,010,087, in order to reduce the bromination of the phenyl nucleus. Thus, these processes have the disadvantage of producing several percent of the phenyl nucleus-brominated compounds.
The inventors have found that only side chain is selectively brominated without production of the by-products, phenyl nucleus-brominated compounds which are not easily separated, when a polyhalogenated ethane is used as a brominating agent in the bromination of the side chain of m-phenoxytoluene as described in Japanese Patent Application No. 66801/1979. However, the reaction is not resulted at a lower temperature of 150.degree. C. or lower in the process of the bromination by heating without a radical initiator or a radiation of an ultraviolet rays and the reaction is remarkably slow at a temperature of 150.degree. to 180.degree. C. to require a long reaction time to attain high conversion of m-phenoxytoluene. These processes are not economical. In the bromination at 200.degree. C. or higher, the reaction is completed for a short time. However, a special reactor is required for an industrial reaction because of high temperature. The reaction mixture obtained by the bromination is colored in black to produce about 10% of high boiling tar as the by-products. It has been required to attain a bromination of the side chain at lower than 200.degree. C. under a reduction of the side-reactions.